The present invention is directed to measurement of density, and in particular to the measurement of the density of a fluid. It is particularly beneficial in industrial applications in which simplicity is desirable and space limitations may be a factor.
Many approaches to measuring fluid density have been proposed and used. One approach that is relatively elegant from a theoretical standpoint is the one described in Soviet Inventors' Certificate No. 1,300,333. That densitometer employs a magnetic bob disposed in a chamber that has piezoelectric sensors at its upper and lower walls. An electric coil draws the ball to the top of the chamber, where the time of its arrival is sensed by the piezoelectric sensor, and the ball is then allowed to drop to the bottom of the chamber, where its arrival is similarly sensed. The current that drives the coil is adjusted so that the time required to reach the top of the chamber equals that required to reach the bottom, and computation circuitry infers the fluid density from the ratio between that current level and the current level similarly arrived at with a fluid of known density. This approach is more suited to laboratory work than to process-control applications, however, since it is relatively slow; it requires many transits through the fluid in order to arrive at the current level that will make the up and down transit times equal.
An approach with more process-control applicability is one in which radiation from a radioactive substance is directed through the fluid sample, and the proportion of the emitted radiation that passes through the fluid sample is taken as an indication of the fluid density. This approach is particularly insensitive to external vibration, but it requires both safety training of personnel and the ability to deal with any resulting health and safety problems.
Another approach employs a plummet that is designed to be positively buoyant with respect to the fluid to be measured. Chains having negative buoyancy are connected between the plummet and reference points in a chamber that contains the fluid; that is, the chamber wall supports each chain at one end, while the other end of each chain is supported by the plummet, which in turn is held down by the weight of the chain. The level at which the plummet end of the chain is held with respect to the level at which it is held by the chamber determines the distribution of the chain weight between the plummet and the chamber wall, and this in turn is determined by the buoyancy of the plummet. Consequently, the density of the fluid can be determined by the height of the plummet. This arrangement has the advantage of simplicity, but it must be made physically large to achieve adequate resistance to vibration, flow effects, and the effects of static friction, which can detract from repeatability and accuracy.
In one of the more widely used approaches, the fluid to be measured flows through a tube, which is caused to vibrate at the natural frequency that results from the tube material, its dimensions, and the fluid that it contains, i.e., the fluid to be measured. The natural frequency depends on the fluid density, which can accordingly be determined by measuring the frequency of the tube vibrations. Such an arrangement is simple in principle, and the accuracy and repeatability reported for such instruments have been good.